The present invention relates to an oscillation circuit capable of oscillating a high frequency wave.
A conventional oscillation circuit disposed in a wireless device is formed of a transistor and a resonance circuit formed of an inductor and a capacitor. When a valuable capacitor is used as the capacitor, the conventional oscillation circuit constitutes a voltage controlled oscillator (VCO) capable of controlling a frequency. Further, the voltage controlled oscillator may be combined with a phase comparator, a loop filter, and a frequency divider to constitute a phase-locked loop (PLL) circuit to be used in a PLL frequency synthesizer.
In recent years, such a conventional oscillation circuit is formed as a part of a semiconductor integrated circuit. When the conventional oscillation circuit is formed as a part of the semiconductor integrated circuit, all components constituting the conventional oscillation circuit may be integrated into the semiconductor integrated circuit. Alternatively, a part of the components constituting the conventional oscillation circuit may be disposed outside the semiconductor integrated circuit (as a peripheral component).
For example, when an oscillation wave frequency becomes lower than few hundred mega Hertz (MHz), it is necessary to provide an inductor having an inductance of few tens nano Henry (nH). In this case, if the inductor is disposed in the semiconductor integrated circuit, it is necessary to provide a large area for installing the inductor, thereby increasing a size of the semiconductor integrated circuit. For this reason, when the oscillation wave frequency becomes lower than few hundred mega Hertz (MHz), the inductor is generally disposed outside the semiconductor integrated circuit. On the other hand, when the oscillation wave frequency becomes higher than one giga Hertz (GHz), it is necessary to provide an inductor having an inductance of about 5 nH. In this case, it is possible to dispose the inductor inside the semiconductor integrated circuit.
Patent Reference 1 has disclosed a conventional oscillation circuit, in which an inductor thereof is disposed as a peripheral component outside a semiconductor integrated circuit. Patent Reference 2 has disclosed a conventional oscillation circuit, in which a capacitor thereof is disposed as a peripheral component outside a semiconductor integrated circuit.
Further, Patent Reference 3 has disclosed a conventional oscillation circuit, in which a capacitor thereof is disposed inside a semiconductor integrated circuit and an inductor thereof is disposed as a peripheral component outside the semiconductor integrated circuit. The inductor is connected to the capacitor as the peripheral component in series.    Patent Reference 1: Japanese Patent Publication No. 2007-110504    Patent Reference 2: Japanese Patent Publication No. 07-131243    Patent Reference 3: Japanese Patent Publication No. 06-132728
In general, the wireless device includes various circuits having various functions such as a reception circuit, a transmission circuit, a phase synchronization circuit, a control circuit and the like, and the various circuits are disposed in the semiconductor integrated circuit. Especially, the reception circuit is provided for receiving a faint signal, so that a signal from the conventional oscillation circuit may cause interference with the reception circuit.
For example, in the conventional oscillation circuit, in which the transistor and the capacitor are disposed inside a semiconductor integrated circuit and only the inductor is disposed outside the semiconductor integrated circuit, when the oscillation wave frequency of 500 Mhz is realized with the inductor of 15 nH, the capacitor has a capacitance of 6.75 pF determined through the following calculation:
  f  =      1          2      ⁢      π      ⁢              LC            where f represents a resonance frequency, L represents the inductance, and C represents the capacitance.
When it is supposed that the conventional oscillation circuit has a voltage amplitude with an effective value of 1 V, the reactance of the inductor becomes 47.1Ω(2πfL=47.1). Accordingly, a high frequency wave current flowing through the conventional oscillation circuit is determined to be 21.2 mA (1/47.1=0.0212), which is a large current corresponding to a power source current of the semiconductor integrated circuit.
When such a large high frequency wave current flows through the conventional oscillation circuit, a conductive material constituting a current path of the conventional oscillation circuit radiates an electromagnetic wave. Accordingly, a board potential of the semiconductor element tends to fluctuate, thereby deteriorating properties of other circuits in the semiconductor integrated circuit. For example, a property of the semiconductor integrated circuit such as reception sensitivity may be deteriorated.
To this end, it may be configured such that an oscillation amplitude may be decreased. When the oscillation amplitude is decreased, however, an extent of purity of the oscillation signal is deteriorated, thereby making it difficult to apply such a measure. Further, an interval between the conventional oscillation circuit and other circuits needs to be increased, or a shield member needs to be disposed between the conventional oscillation circuit and other circuits. However, such a measure increases an area as well as a cost of the semiconductor integrated circuit, thereby making the application thereof difficult.
In view of the problems described above, an object of the present invention is to provide an oscillation circuit capable of solving the problems of the conventional oscillation circuit. In the present invention, it is possible to prevent properties of other circuits disposed in a semiconductor integrated circuit from deteriorating when an oscillation circuit outputs an oscillation signal having an oscillation wave frequency of few hundred mega Hertz.
Further objects and advantages of the invention will be apparent from the following description of the invention.